Electronic switching output stages may be tested for defects by diagnostic methods. Electronic switching output stages may be tested for a drop in load in which no load current occurs, for an underload in which the load current that occurs is too low, for an overload in which the load current that occurs is too high, and for a short circuit to ground and a short circuit to the power supply. In addition, the electronic switching output stage is usually diagnosed to determine whether there is an excess temperature. In the diagnostic method presented here, a distinction is made between high-side switches (HS) and low-side switches (LS).
Within the scope of the diagnostic methods listed above, the fault may be detected under the prerequisites indicated below:
With respect to the high-side output stage, diagnosis for a drop in load with the output stage turned on or off is performed by applying a test current or applying a test potential. The diagnosis for a drop in load is performed by a measurement at a shunt and/or by a voltage measurement on the load.
Diagnosis for an underload of the electronic switching output stage is usually performed with the output stage on by measuring the voltage drop at a shunt.
Diagnosis for an overload is usually also performed with the output stage on by measuring the voltage drop at a shunt and/or measuring an on-resistance of the output stage.
Diagnosis of the electronic switching output stage for short circuit to ground (low resistance or high resistance) is usually performed with the output stage on via a voltage drop at the shunt and/or by ascertaining an on-resistance of the output stage. The diagnosis with regard to a short circuit to power supply (Ubat), whether it is low resistance or high resistance, is usually performed with the output stage off by applying a test current or a test potential on the basis of a voltage measurement at the load, whereas within the scope of the diagnostic methods listed above, an excess temperature measurement is always performed usually via a temperature measurement by ascertaining the diode voltage. Alternatively, a temperature-dependent resistance may be measured.
With respect to the low side of the electronic switching output stage, a drop in load is diagnosed with the output stage turned on or off by applying a test current or a test potential. The measurement method used is usually to measure the voltage drop at a shunt and/or to measure an on-resistance of the output stage and to measure the voltage of the load.
Diagnosis with regard to the underload applied to the electronic switching output stage is performed with the output stage on by measuring the voltage drop at a shunt. The same thing also applies to diagnosis of an overload. A short circuit to ground (low resistance, high resistance) is diagnosed with the output stage off by applying a test current or a test potential, usually by a voltage measurement at the load. A short circuit to power supply (Ubat), whether low resistance or high resistance, is diagnosed with the output stage on by measuring the voltage at the shunt, whereas as in the case of the excess temperature measurement of the high side of the output stage, an excess temperature measurement is always performed for which purpose a temperature measurement by ascertaining the diode voltage or a temperature-dependent resistance may be used.
The diagnostic methods described above have the disadvantage in the case of clocked output stages and inductive loads that when the output stage is off, the voltage measurement at the load functions only to a limited extent during clocking. This means that, for example, high-resistance shunts parallel to the output stage are not able to be detected because the inductive load determines the potential at the load in the free-wheeling phase with the free-wheeling diode and the shunt.
German patent document DE 198.51 732 A1 discusses a method and a device for monitoring at least one current regulating stage. According to the method from DE 198 51 732 A1, at least one current regulating stage for an electrical consumer is monitored by a device which includes at least one switching arrangement and one current regulator. A current, which is determined by a pulse duty factor, flows through the at least one consumer, the current being able to be regulated to a setpoint value by the current regulator. For monitoring, the pulse duty factor and/or a variable derived from the pulse duty factor are/is compared with a threshold value, and when there is a deviation from this threshold value, the existence of a fault is detected. Instead of the pulse duty factor, a ratio between the pulse duty factor and the setpoint value may be formed and compared with a comparison value. Starting from the pulse duty factor and the voltage, a current value may be determined and compared with a comparison value.
The diagnostic method of an electronic switching output stage, which is known from the related art and is outlined above, as well as the approach from DE 198 51 732 A1, are both subject to the disadvantage that during pulse-width-modulated operation of electronic switching output stages, high-resistance short circuits to the power supply or to ground are detectable only to a limited extent.